The heart emits electrical signals as a by-product of the propagation of the action potentials that trigger depolarization of heart fibers. An electrocardiogram (ECG) measures and records such electrical potentials to visually depict the electrical activity of the heart over time. Conventionally, a standardized set format 12-lead configuration is used by an ECG machine to record cardiac electrical signals from well-established traditional chest locations. Electrodes at the end of each lead are placed on the skin over the anterior thoracic region of the patient's body to the lower right and to the lower left of the sternum, on the left anterior chest, and on the limbs. Sensed cardiac electrical activity is represented by PQRSTU waveforms that can be interpreted post-ECG recordation to derive heart rate and physiology. The P-wave represents atrial electrical activity. The QRSTU components represent ventricular electrical activity.
An ECG is a tool used by physicians to diagnose heart problems and other potential health concerns. An ECG is a snapshot of heart function, typically recorded over 12 seconds, that can help diagnose rate and regularity of heartbeats, effect of drugs or cardiac devices, including pacemakers and implantable cardioverter-defibrillators (ICDs), and whether a patient has heart disease. ECGs are used in-clinic during appointments, and, as a result, are limited to recording only those heart-related aspects present at the time of recording. Sporadic conditions that may not show up during a spot ECG recording require other means to diagnose them. These disorders include fainting or syncope; rhythm disorders, such as tachyarrhythmias and bradyarrhythmias; apneic episodes; and other cardiac and related disorders. Thus, an ECG only provides a partial picture and can be insufficient for complete patient diagnosis of many cardiac disorders.
The inadequacy of conventional, short-term, ECG recordings is particularly apparent in the case of sleep apnea, a type of sleep disorder that affects a patient's breathing during sleep and may also impact the patient's cardiac activity. ECG monitoring alone may not be useful in diagnosing the condition due to a natural heart rate reduction during sleep. As a patient enters non-rapid eye movement (NREM) sleep, the patient experiences physiological changes due to a withdrawal of activity of the patient's sympathetic nervous system. As a result, even healthy people may experience sinus bradyarrhythmia during sleep, and ECG monitoring alone may not always reveal whether the bradyarrhythmia is naturally-occurring or is caused by a pathological condition, such as an apneic episode. Furthermore, if the patient experiences other types of arrhythmias during sleep, without having a telemetry of the patient's air flow, the flow of air in and out of the patient's lungs during breathing, or another indicator of the patient's respiration, the physician may not be always be able to determine if an arrhythmia is a result of a sleep apnea episode or of some other morbidity. However, considering that cardiac manifestations of sleep apnea are most apparent at night, a short-term ECG monitoring done in a clinic during business hours may not reveal even the presence of the cardiac arrhythmia.
Diagnostic efficacy can be improved, when appropriate, through the use of long-term extended ECG monitoring coupled to pulmonary measures. Recording sufficient ECG and related physiology over an extended period is challenging, and often essential to enabling a physician to identify events of potential concern. A 30-day observation period is considered the “gold standard” of ECG monitoring, yet achieving a 30-day observation day period has proven unworkable because such ECG monitoring systems are arduous to employ, cumbersome to the patient, and excessively costly. Ambulatory monitoring in-clinic is implausible and impracticable. Nevertheless, if a patient's ECG and pulmonary measures could be recorded in an ambulatory setting, thereby allowing the patient to engage in activities of daily living, the chances of acquiring meaningful information and capturing an abnormal event while the patient is engaged in normal activities becomes more likely to be achieved.
For instance, the long-term wear of ECG electrodes is complicated by skin irritation and the inability ECG electrodes to maintain continual skin contact after a day or two. Moreover, time, dirt, moisture, and other environmental contaminants, as well as perspiration, skin oil, and dead skin cells from the patient's body, can get between an ECG electrode, the non-conductive adhesive used to adhere the ECG electrode, and the skin's surface. All of these factors adversely affect electrode adhesion and the quality of cardiac signal recordings. Furthermore, the physical movements of the patient and their clothing impart various compressional, tensile, and torsional forces on the contact point of an ECG electrode, especially over long recording times, and an inflexibly fastened ECG electrode will be prone to becoming dislodged. Notwithstanding the cause of electrode dislodgment, depending upon the type of ECG monitor employed, precise re-placement of a dislodged ECG electrode maybe essential to ensuring signal capture at the same fidelity. Moreover, dislodgment may occur unbeknownst to the patient, making the ECG recordings worthless. Further, some patients may have skin that is susceptible to itching or irritation, and the wearing of ECG electrodes can aggravate such skin conditions. Thus, a patient may want or need to periodically remove or replace ECG electrodes during a long-term ECG monitoring period, whether to replace a dislodged electrode, reestablish better adhesion, alleviate itching or irritation, allow for cleansing of the skin, allow for showering and exercise, or for other purpose. Such replacement or slight alteration in electrode location actually facilitates the goal of recording the ECG signal for long periods of time.
Conventionally, Holter monitors are widely used for long-term extended ECG monitoring. Typically, they are used for only 24-48 hours. A typical Holter monitor is a wearable and portable version of an ECG that include cables for each electrode placed on the skin and a separate battery-powered ECG recorder. The cable and electrode combination (or leads) are placed in the anterior thoracic region in a manner similar to what is done with an in-clinic standard ECG machine. The duration of a Holter monitoring recording depends on the sensing and storage capabilities of the monitor, as well as battery life. A “looping” Holter monitor (or event) can operate for a longer period of time by overwriting older ECG tracings, thence “recycling” storage in favor of extended operation, yet at the risk of losing event data. Although capable of extended ECG monitoring, Holter monitors are cumbersome, expensive and typically only available by medical prescription, which limits their usability. Further, the skill required to properly place the electrodes on the patient's chest hinders or precludes a patient from replacing or removing the precordial leads and usually involves moving the patient from the physician office to a specialized center within the hospital or clinic. Also, Holter monitors do not provide information about the patient's air flow, further limiting their usefulness in diagnosing the patient.
The ZIO XT Patch and ZIO Event Card devices, manufactured by iRhythm Tech., Inc., San Francisco, Calif., are wearable stick-on monitoring devices that are typically worn on the upper left pectoral region to respectively provide continuous and looping ECG recording. The location is used to simulate surgically implanted monitors. Both of these devices are prescription-only and for single patient use. The ZIO XT Patch device is limited to a 14-day monitoring period, while the electrodes only of the ZIO Event Card device can be worn for up to 30 days. The ZIO XT Patch device combines both electronic recordation components, including battery, and physical electrodes into a unitary assembly that adheres to the patient's skin. The ZIO XT Patch device uses adhesive sufficiently strong to support the weight of both the monitor and the electrodes over an extended period of time and to resist disadherance from the patient's body, albeit at the cost of disallowing removal or relocation during the monitoring period. Moreover, throughout monitoring, the battery is continually depleted and battery capacity can potentially limit overall monitoring duration. The ZIO Event Card device is a form of downsized Holter monitor with a recorder component that must be removed temporarily during baths or other activities that could damage the non-waterproof electronics. Both devices represent compromises between length of wear and quality of ECG monitoring, especially with respect to ease of long term use, female-friendly fit, and quality of atrial (P-wave) signals. Furthermore, both devices do not monitor the patient's air flow, further limiting their usefulness in diagnosing the patient.
While portable devices that combine respiratory and cardiac monitoring exist, these devices are also generally inadequate for long-term monitoring due to their inconvenience and restraint that they place on the patient's movements. For example, SleepView monitor devices, manufactured by Cleveland Medical Devices Inc. of Cleveland, Ohio, require a patient to wear multiple sensors on the patient's body, including a belt on the patient's chest, a nasal cannula, and an oximetry sensor on the patient's finger, with these sensors being connected by tubing and wires to a recording device worn on the belt. Having to wear these sensors throughout the patient's body limits the patient's mobility and may be embarrassing to the patient if worn in public, deterring the patient from undergoing such a monitoring for an extended period of time.
Therefore, a need remains for a self-contained personal air flow monitor capable of recording both air flow data, other respiratory data such as respiratory rate and effort, and ECG data, practicably capable of being worn for a long period of time in both men and women, and capable of recording atrial signals reliably.
A further need remains for a device capable of recording signals ideal for arrhythmia discrimination, especially a device designed for atrial activity recording, as the arrhythmias are coupled to the associated pulmonary problems common to sleep apnea and other respiratory disorders.